1. Field of the Invention
The present invention relates to a starting clutch provided coaxially mounted on a crankshaft and being capable of transmitting a motive power of the crankshaft to a transmission device via a gear.
2. Description of Background Art
A starting clutch is known wherein an automatic centrifugal clutch includes a clutch inner fitted integrally with the crankshaft and a clutch outer rotatably supported by the crankshaft that come into and out of contact with each other according to a rotational speed.
For example, a clutch device disclosed in JP-A-2002-48150 is illustrated in FIG. 6 wherein a clutch inner 02 is configured in such a manner that a disk-shaped drive plate 04 is secured and supported to a drive plate boss 03 which is fitted to a portion of a crankshaft 01 near an end thereof. A clutch shoe 06 is provided on a pin 05 projecting from the drive plate 03 so as to be capable of swinging by way of a centrifugal force.
On the other hand, a clutch outer 010 includes a clutch housing 012 which forms a cup shape by an annular portion which covers the outer periphery of the drive plate 04 and a hollow disk portion which covers the center side of the side surface of the crankshaft. A cylindrical clutch outer boss 011 holds the clutch housing 011 integrally and is rotatably fitted on the crankshaft 01.
A one-way clutch 020 is interposed between a cylindrical portion 03a of the drive plate boss 03 of the clutch inner 02 which extends toward the center side of the crankshaft and an opening cylindrical portion 011a of the clutch outer boss 011 of the clutch outer 010.
An end edge of a bearing cylindrical portion 011b of the clutch outer boss 011 which is rotatably fitted to the crankshaft 01 continues from the opening cylindrical portion 011a having a large diameter via a shoulder portion 011c. 
Therefore, the portion of the clutch inner 02, being fitted to the crankshaft and integrally fixed thereto, which receives a thrust force of the clutch outer 010 in the axial direction thereof is the drive plate boss 03 of the clutch inner 02, and the opened end surface of the bearing cylindrical portion 011b of the clutch outer 010 comes into abutment with the opened end surface of the cylindrical portion 03a of the drive plate boss 03 which is opposed thereto to receive the thrust.
Since the thrust receiving surface of the cylindrical portion 03a of the drive plate boss 03 is located on a relatively smaller diameter portion thereof which is on the axial center side with respect to the one-way clutch 020, it is necessary to increase the outer diameter of the cylindrical portion 03a of the drive plate boss 03 in order to secure a sufficient surface area for receiving the thrust.
Therefore, the diameter of the one-way clutch 020 is increased. Thus, the size of the entire internal combustion engine is also affected.